Since organic EL devices are self-luminescent devices, they are bright and excellent in visibility as compared with liquid-crystalline devices and capable of giving clear display. Therefore, the organic EL devices have been actively studied.
In 1987, C. W. Tang et al. of Eastman Kodak Company put an organic EL device using organic materials into practical use by developing a device having a multilayered structure wherein various roles are assigned to respective materials. They formed a lamination of a fluorescent material capable of transporting electrons and an organic material capable of transporting holes, so that both charges are injected into the layer of the fluorescent material to emit light, thereby achieving a high luminance of 1000 cd/m2 or more at a voltage of 10 V or lower (see e.g., Patent Documents 1 and 2).
Patent Document 1: JP-A-8-48656
Patent Document 2: Japanese Patent No. 3194657
To date, many improvements have been performed for practical utilization of the organic EL devices, and high efficiency and durability have been achieved by an electroluminescent device wherein an anode, a hole-injecting layer, a hole-transporting layer, an emitting layer, an electron-transporting layer, an electron-injecting layer, and a cathode are sequentially provided on a substrate, to further segmentalize various roles of the multilayered structure (see e.g., Non-Patent Document 1).
Non-Patent Document 1: Japan Society of Applied Physics Ninth Workshop Preprint, pp. 55-61 (2001)
Moreover, for the purpose of further improvement of luminous efficiency, utilization of triplet exciton has been attempted and utilization of a phosphorescent material has been investigated (see e.g., Non-Patent Document 2).
Non-Patent Document 2: Japan Society of Applied Physics Ninth Workshop Preprint, pp. 23-31 (2001)
The emitting layer can be also prepared by doping a charge-transporting compound, generally called a host material, with a fluorescent material or a phosphorescent material. As described in the above-mentioned Non-Patent Document, the choice of the organic materials in organic EL devices remarkably affects various properties such as efficiency and durability of the devices (see Non-Patent Document 2).
In the organic EL devices, the charges injected from the both electrodes are recombined in the emitting layer to attain light emission. In order to obtain an organic EL device having a high efficiency, a low driving voltage and a long life, it is necessary to provide a device with an excellent carrier balance, in which electrons or holes can be efficiently injected and transported and the both can be efficiently recombined.
As a hole-injecting material which is used for the organic EL devices, phthalocyanines such as copper phthalocyanine (hereinafter referred to as “CuPc”) were proposed at the beginning (see, e.g., Patent Document 3). However, since the phthalocyanines have absorption in the visible range, materials having a phenylenediamine structure have become widely used (see, e.g., Patent Document 4). On the other hand, as a hole-transporting material, arylamine based materials containing a benzidine skeleton have been used (see, e.g., Patent Document 5).
Patent Document 3: U.S. Pat. No. 4,720,432
Patent Document 4: JP-A-8-291115
Patent Document 5: Japanese Patent No. 3529735
A representative light-emitting material, tris(8-hydroxyquinoline)aluminum (hereinafter referred to as “Alq3”) is commonly used as an electron-transporting material. However, as compared with a hole mobility which generally used hole-transporting materials have, an electron mobility which Alq3 has is low, and Alq3 has a work function of 5.8 eV so that it cannot be considered that Alq3 has sufficient hole-blocking ability. Therefore, a part of the holes passes through the emitting layer, resulting in lowering of the efficiency.
Furthermore, for the purpose of efficiently achieving hole injection or electron injection from an anode or a cathode into the emitting layer, there has been developed a device in which a value of ionization potential and a value of electron affinity which material has are set up in stages and two or more layers are laminated with respect to each of a hole-injecting layer and an electron-injecting layer (see, e.g., Patent Document 6). However, it cannot be said that the used materials are sufficient in all of luminous efficiency, driving voltage and device life.
Patent Document 6: JP-A-6-314594
In usual, since the hole-transporting layer is an extremely thin film, it is influenced by roughness of the surface of an ITO electrode, and there is a high probability of occurrence of defective products in the prepared device due to the occurrence of a short circuit or the like. When a film thickness of the hole-transporting layer is made thick, the roughness of the surface of the ITO electrode can be hidden, and it can reduce the probability of occurrence of defective products in the prepared device. However, when the film thickness of the hole-transporting layer is made thick, the driving voltage arises and exceeds a practical driving voltage.
For the purposes of improving device characteristics of organic EL devices and enhancing a yield of the device preparation, it has been demanded a device capable of achieving efficient recombination of holes and electrons and having a high luminous efficiency, a low driving voltage and a long life, by combining materials which are excellent in an injection or transportation performance of holes or electrons, and in stability and durability in a thin film.
Also, for the purpose of improving device characteristics of organic EL devices, it has been demanded a device with a good carrier balance and having a high efficiency, a low driving voltage and a long life, by combining materials which are excellent in an injection and transportation performance of holes or electrons, and in stability and durability in a thin film.